1. Field of the Invention
The present invention relates to an optical scanning apparatus and a color image forming apparatus using the same. The present invention is suitable for use in a color image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotographic process.
2. Description of the Related Art
Hitherto, an optical scanning apparatus has been used in an image forming apparatus such as a digital copying machine or a laser beam printer (LBP). In the optical scanning apparatus of this type, opposed scanning units in which a plurality of scanning units is arranged so as to oppose each other with an optical deflector interposed therebetween are used as a means for downsizing the entire apparatus. The optical scanning apparatus using the opposed scanning units performs scanning with a plurality of light beams by means of the same optical deflector and guides the light beams to different surfaces to be scanned by means of imaging optical systems arranged on both sides of the optical deflector. Various optical scanning apparatuses using such a method have conventionally been proposed (refer to Japanese Patent Application Laid-Open Nos. H10-148781, H09-197309, and H09-197315).
Japanese Patent Application Laid-Open No. H10-148781 describes an optical scanning apparatus in which a common optical deflector causes light beams emitted from two light source units to be deflected and reflected from different deflecting surfaces, and two imaging optical systems provided so as to oppose each other with the optical deflector interposed therebetween cause the deflected light beams to form spot images on different surfaces to be scanned, thus performing image formation.
A problem with such an arrangement that the optical axes of the imaging optical systems are approximately parallel to each other with the optical deflector interposed therebetween is that undesirable light generated by one imaging optical system passes over the optical deflector to enter the other imaging optical system and reaches the surface to be scanned, thus causing image-quality deterioration. The undesirable light is stray light and ghost.
As illustrated in FIG. 3B of Japanese Patent Application Laid-Open No. H10-148781, the undesirable light is generated from a region arranged slightly outside the diameter of an inscribed circle of the optical deflector. In Japanese Patent Application Laid-Open No. H10-148781, as a countermeasure against undesirable light, a part of a housing member of the optical deflector is formed as a light blocking wall.
As illustrated in FIG. 1 of Japanese Patent Application Laid-Open No. H10-148781, an additional light blocking member (light blocking plate) is provided in a region that is not covered by the housing member. The housing member of Japanese Patent Application Laid-Open No. H10-148781 is a cover of a polygon mirror which is provided for the purpose of dirt prevention and wind noise reduction in a rotational polygon mirror used as the optical deflector. As another countermeasure against undesirable light, Japanese Patent Application Laid-Open No. H09-197309 discusses an optical scanning apparatus which has a common optical deflector configured to cause light beams to be obliquely incident on deflecting surfaces from a sub-scanning direction so that incident optical systems arranged so as to oppose each other with the optical deflector interposed therebetween have different oblique incidence angles. According to this configuration, since undesirable light reflected from one imaging optical system reaches the other imaging optical system arranged on the opposite side of the optical deflector at different height and angle, the undesirable light will not pass through the imaging optical system to reach the surface to be scanned. Although Japanese Patent Application Laid-Open No. H09-197309 describes an overfilled optical system as an example, the same advantage is obtained in an underfilled optical system.
In Japanese Patent Application Laid-Open No. H09-197135, as a countermeasure against undesirable light in the overfilled optical system, two light blocking members are arranged outside a circumscribed circle of the overfilled optical system. The distance between opposing ends of both light blocking members is set to be smaller than the diameter of an inscribed circle thereof.
As described in Japanese Patent Application Laid-Open No. H09-197315, the light blocking members for blocking undesirable light are arranged outside the circumscribed circle of the rotational polygon mirror, and a line connecting the opposing ends of the light blocking members extends so as to intersect the diameter of the inscribed circle of the rotational polygon mirror. However, in the vicinity of the deflecting surface, light beams entering from a light source unit or scanning light beams reflected from the optical deflector appear. For this reason, there is a problem in that it is difficult to dispose the light blocking members so as to completely block the undesirable light while not interfering with these light beams. Moreover, in order to detect the writing position of a scanning optical system, a writing position detecting optical system (BD optical system) is typically arranged close to the incident optical systems. For this reason, there is another problem in that the angle between the incident optical systems that make light beams incident on the deflecting surfaces increases, and the space for arranging the light blocking members decreases, thus making the arrangement further difficult.